The invention concerns a translation control device usable among other things for focusing an optical instrument on a spacecraft, for example, and more generally any highly accurate rotational coupling device.
Optical instruments on board satellites, such as telescopes, for example, in some cases include focusing mechanisms in order to improve the quality of the images received. The function of such mechanisms is to optimally position one or more lenses disposed in the optical field in order to focus the light beam correctly.
The lens is to this end moved in rectilinear displacement along the optical axis of the light beam by a motorized device in practice consisting of an electrical rotating stepper motor, the output shaft of which cooperates with a lead screw type translation control system.
The lens also cooperates with a pure translation guidance system along the optical axis. The system must be highly accurate as it must avoid any misalignment of the lens relative to the optical axis, as a result of rotation and/or transverse displacement.
The motorized device controlling the axial displacement of the lens must, on the one hand, guarantee the integrity of the guidance function so as to avoid compromising the optical performance of the mechanism since it must not result in any angular or transverse offset of the optical axis. On the other hand, the motorized device must provide an adequate motor drive margin without requiring excessive onboard power, as it must not generate disturbing resisting forces or torques.
The displacement control device must, therefore, enable application to the lensholder assembly of a force F parallel to the optical axis without generating any disturbing mechanical stresses, such as forces transverse to the optical axis, tilting moments, etc.
Disturbing mechanical stresses appear immediately when there is any misalignment, even less than one degree, between the motor axis and the axis of the lead screw device. Misalignment is difficult to prevent because of manufacturing tolerances, assembly defects, temperature variations that can cause differential thermal expansion, and so on.